Chateau has an image maximum image width in the posts of 529px. That means anything larger than that will have a new image at 529px created by wordpress for use in the post. The larger the original image is, the less quality the reduced sized image will have.

With your images, in Photoshop always do the "export for web & devices" from the file menu. That will optimize the images for the web at 72 dpi which is the web standard. It will also save them in sRGB format automatically which is what the web standard is. It does no good to have anything of higher dpi since the web and web browsers will ignore anything larger and do 72dpi when they display them.

The exception to this is Safari which will actually show an image at the dpi of the image, no matter what that is. Safari is the exception though, not the rule. I know of no other browser that does that (Safari will even display photoshop PSD files that have been uploaded to the web where no other browser will).

Create your images and size them to no more than 800px wide so that wordpress does not have to do a lot of work to reduce them, do the export for web and devices from within photoshop and upload that image.